Thermoformed parts are typically formed by the action of a forming die upon a continuous thermoplastic web and the parts, after receiving their form, remain in the web and are led to a second station where they are trimmed from the web by the action of a trimming die. The web carrying the parts is indexed into registry with the trim die. Due to the nature of the thermoformable substance of the web, e.g. nonuniform shrinkage, variation in index length occurs. Even between consecutive ranks of formed parts, variations of the order of 0.005 inches are not atypical. To provide accurate positioning, the parts must be indexed according to measurements performed close to the trim die, preferably on the part to be trimmed, but the indexing means must be clear of the die for the trimming operation. Typically this indexing is done by the action of reciprocating fingers. These engage the web, draw the web a predetermined distance into alignment with the die, and return for a further stroke. The necessary size of the fingers and the required acceleration and deceleration limit the speed of operation. Also, the width of the finger represents unusable area of the plastic web.
Experience has shown that trim machines of this type are capable of speeds of less than 100 cycles per minute for sustained operation. Faster speeds, though occasionally possible, usually involve vibrations in work conditions that require frequent maintenance of the machine, and also introduce inaccuracies in the trimming of the parts.
There have been prior efforts to substitute other drive devices for the fingers. One system has attempted to use rotary nipping devices at each side of the web. These have not found wide-spread use, partly because of problems of jamming of the feed. Die pins, used to locate and position metal webs in the distinctly different field of metal stamping and punching operations may not be employed in the thermoforming field due to the nonuniform shrink characteristics of the thermoformable web.
It is an objective of this invention to provide a device to provide an improved reliable trim press capable of high speed, accurate operation on thermoformed webs.